Method for thermal surface treatment in a continuous casting machine and relative device

Abstract: Method for thermal surface treatment in line in a continuous casting machine associated with furnaces to heat hot-charge blooms, the method being applied to fine-grain structural steels and being suitable to obviate the precipitation of compounds of aluminium, vanadium, niobium and the like and to eliminate or at least to reduce greatly the surface faults due to tension, the method being carried out in a continuous casting line comprising at least a mould (13), a secondary cooling chamber (14), an extraction and straightening assembly (15) and a shearing assembly (17), the method being applied in cooperation with the extraction and straightening assembly (15) and including an intense, concentrated cooling of the surface of a bloom (19) passing through by means of a cooling fluid under pressure, which is water-based and is sprayed by a plurality of sprayer nozzles (18), the cooling being adapted to the dimensions of the bloom (19) and being such as to produce a surface temperature between about 400°C and about 900°C after the natural tempering caused by the hot core of the bloom (19); and a device suitable to carry out the above method and including a plurality of sprayer nozzles (18) arranged about the circumference of the bloom (19) and facing the bloom (19), the sprayer nozzles (18) being fed by means (22) which deliver a water-based cooling fluid under pressure and which are associated at least with means (23) that regulate the pressure.

Description: This invention concerns a method for thermal surface treatment in a continuous casting machine and also concerns a device suitable to carry out this method, as set forth in the main claims. The method and relative device according to the invention are employed in the metallurgical field and in particular on continuous casting machines to perform continuous controlled cooling of the bloom being fed. The invention is applied to the treatment of blooms, billets or slabs having a square, round, rectangular or polygonal cross-section and consisting of fine-grain structural steels. In the description that follows, we shall use only the word "blooms" for descriptive convenience but shall mean thereby that the device is applied also to billets and slabs having any of the above cross-sections. The invention is applied advantageously, but not only, to the treatment of the following families of steels: steels for structural uses, steels for cold processing, steels for forging, steels for cementation, hardening and tempering steels and surface hardening steels. Moreover the invention is applied in particular to steels having an appreciable content of aluminium. Continuous casting plants use the technique of carrying out a quenching operation on moving blooms before the blooms are fed into a furnace with a hot charge. The quenching operation in continuous casting plants is performed downstream of the shearing assembly consisting, for instance, of a shears or oxygen-cutting torches, depending on the thickness of the bloom; this shearing assembly is installed downstream of the extraction and straightening segment of the plant. The quenching operation, however, entails a series of drawbacks linked to the fact that the bloom arriving at the quenching station has too low a temperature. Moreover, in the plants of the state of the art, when the quenching operation is carried out on the sheared bloom, there is a considerable difference of temperature between its two ends, and this difference of temperature results in a lack of structural homogeneity with consequent faults in the bloom and/or problems in the plant downstream. There is also the method of cooling the blooms in air, which is advantageously a forced draught at a temperature below 600°C, so as to cause conversion of the austenite in their surface layer before charging them into the furnace. This entails in practice a cooling of the bloom, which reduces a great deal of the energy saving linked to the hot charging process. The state of the art includes an auxiliary cooling method which employs a device arranged upstream of the extraction and straightening assembly. This method was devised essentially for carrying out thermal soft reduction, namely to reduce the segregation of the carbon on the central axis of the bloom or billet but not to reduce and eliminate the problems of hot shortness, which are characteristic of a hot charge of steels containing aluminium. Moreover, this method is applied mainly to steels with a high carbon content and with very low percentages of aluminium. Furthermore, in the method of the state of the art the surface temperature of the billet or bloom at the straightening assembly is about 800°C. This temperature may be too high to compensate the drawing stresses arising from the extraction and straightening, for these stresses entail a possible occurrence of transverse cracks in the skin of the bar. Besides, where the steels have a high aluminium content, the auxiliary cooling of the state of the art may be not enough to eliminate the formation of surface faults, for in this case such formation is due not only to the high temperature but also to precipitation of the nitrides. JP-A-63-160765 discloses a device for preventing generation of harmful cutting burrs at the time of cutting an ingot in a continuous casting line, wherein a small-diameter roll is disposed in contact with the ingot for rotating according the rotating speed of the ingot. The casting speed of the ingot is detected by a casting speed detector when the casting speed of the ingot increases. In addition to a secondary chamber to cool the ingot immediately outside the mould, cooling sprays are provided before the shear to selectively cool the ingot when it has to be cut. The cooling water flow rate in the cooling sprays is increased by a cooling water flow rate regulator with an increase in the speed, by which the temperature of the ingot at the time of cutting is lowered. Since the brittleness of the ingot is improved at the time when the ingot is cut by the shear, the generation of the harmful cutting burrs is prevented. This document does not show nor suggests any surface quenching treatment on the outer layer of the ingot. The present applicants have designed, tested and embodied this invention to overcome the shortcomings of the state of the art and to achieve further advantages. This invention is set forth and characterised in the respective main claims, while the dependent claims describe variants of the idea of the main embodiment. The purpose of this invention is to provide a method and device which create in the moving bloom an outer fine-grain layer having a structure of great strength and toughness. According to the invention the thermal treatment is carried out in the casting line immediately downstream of the extraction and straightening assembly and upstream of the shearing assembly. According to a variant, the thermal treatment is carried out upstream of the extraction and straightening assembly and within the secondary cooling chamber of the continuous casting machine. According to another variant, in particular where a short secondary cooling chamber is included which does not reach the vicinity of the extraction and straightening assembly, the thermal treatment is carried out outside the secondary cooling chamber of the continuous casting machine and in a position between that chamber and the extraction and straightening assembly. According to yet another variant, the thermal treatment is carried out either upstream of the extraction and straightening assembly or downstream of the extraction and straightening assembly and upstream of the shearing assembly. The device according to the invention makes it possible to have, downstream of the shearing assembly, fine-grain blooms with a surface temperature between 400°C and 900°C and to eliminate occurrences of hot shortness caused by precipitation of aluminium nitride or vanadium carbonitride or niobium carbonitride. This enables the downstream heating furnaces to be charged with blooms in a hot charge condition. The invention therefore makes possible a reduction of the cycle of heating the blooms charged hot into the furnaces, with an evident saving of energy. These blooms can thus be brought quickly to the required temperature on the basis of the type of steel by using a speed of heating which may reach 500°C per hour. The invention also reduces the surface faults which are due to tension and which develop during the cooling of the blooms in air. The need is therefore also eliminated of having to add nitrogen fixation elements such as titanium and vanadium, which are normally added to prevent formation of the above faults due to tension. This elimination is due to the fact that by means of the invention it is possible to obtain an outer layer even some centimetres thick, but normally at least one centimetre thick, and this outer layer is able to retain and absorb in practice any tension. A further application of the invention provides the so-called "thermal soft reduction" process where at the outlet of the straightening assembly there is a liquid fraction in the core of the bloom, thus reducing the central segregation. The device according to the invention comprises a compact series of rows of sprayer nozzles, which are installed on the continuous casting machines so as to cool continuously the bloom which passes through the sprayer nozzles. These sprayer nozzles are arranged about the periphery of the bloom being fed and are aimed at the bloom in such a way that the whole surface of the bloom is lapped by the jets of cooling fluid. These sprayer nozzles are fed with a cooling fluid under pressure, which is generally water and of which the pressure and rate of flow can be adjusted as desired to suit the type of steel, the dimensions of the cross-section of the bloom and the speed of feed of the bloom. Moreover, the rate of flow and the pressure at the sprayer nozzles are altered also according to the depth of the outer layer to be quenched. According to a variant the means to adjust the sprayer nozzles make possible a differentiated adjustment of the pressure and/or rate of flow at the various sprayer nozzles for special processing requirements. The localised and concentrated cooling on the surface of the bloom causes a surface quenching of the continuously cast bloom at the temperature of departure from the straightening step, whereas the core of the bloom undergoes a much gentler cooling and stays at a substantially constant temperature. Next, the heat of the core of the bloom causes self-tempering of the quenched outer zone and leads, on the basis of the final temperature, to a sorbitic structure, or a structure of ferrite and carbides, or a fine-grain austenitic structure. According to a variant of the invention at least one insulated and possibly heated hood is included downstream of the device positioned between the extraction and straightening assembly and the shearing assembly and accelerates the self-tempering of the quenched outer zone, thus assisting the subsequent shearing operations carried out with a shears or oxygen-cutting torches according to the dimensions of the bloom. The temperature reached by the surface of the bloom during the self-tempering phase can be varied to suit the type of steel and the properties to be achieved. The device according to the invention includes advantageously temperature measurement means fitted upstream and downstream respectively, which measure the temperature of the bloom entering and leaving the device. These temperature measurement means associated also with means measuring the speed of feed of the bloom govern advantageously a controlling, programming and governing unit, which regulates the pressure and rate of flow at the sprayer nozzles. This controlling, programming and governing unit comprises advantageously storage means containing technological data relating to the various types of steels and the working parameters of the device according to the invention. The storage means associated with the controlling, programming and governing unit contain in particular the data relating to the thickness of the bloom to be cooled and the temperature to which that thickness has to be brought both in the cooling and in the successive tempering step. The controlling, programming and governing unit is associated advantageously with data input means comprising a keyboard, for instance. The surface temperature of the bloom entering the device according to the invention depends on the parameters of the continuous casting machine. According to a variant the device positioned downstream of the extraction and straightening assembly and also the device positioned upstream thereof, at least where the latter device is outside the secondary cooling chamber of the continuous casting machine, include advantageously, both upstream and downstream, a series of compressed-air nozzles which form a wall of air acting as a door for the entry and exit respectively of the bloom into and out from the device. These walls of air have the task of preventing the departure of water from the device and of minimising the formation of steam released from the device. The outlet wall of air has the task also of eliminating water which tends to stay on the upper surface of the bloom leaving the device and which would lead to localised and uncontrolled undercooling of the surface of the bloom with a resulting lack of homogeneity in the cooling of the bloom. According to another variant an aspiration hood is fitted in cooperation with the device according to the invention so as to aspirate and remove the steam generated. This aspiration hood may be not included where the device is inside the secondary cooling chamber of the continuous casting machine. In fact, in this case the device cooperates with the means that aspirate and contain the steam present in the secondary cooling chamber. The device enables a method of cooling blooms to be achieved whereby the rate of flow and pressure of the cooling fluid delivered by the sprayer nozzles are regulated according to the properties of the steel, the speed of feed of the bloom and the temperature of the bloom. This cooling method enables a precise layer cooled to the desired temperature to be obtained, this layer being then tempered according to a determined curve. The attached figures are given as a non-restrictive example and show some preferred embodiments of the invention as follows:- Fig.1a is a diagram of a continuous casting plant in which a device according to the invention is installed; Fig.1b shows partly a variant of the plant of the Fig.1a; Fig.2 is a diagram in an enlarged scale of a lengthwise section of the device; Fig.3a shows diagrammatically a cross-section of the device; Fig.3b shows a variant of the device of Fig.3a; Fig.3c shows diagrammatically a possible cross-section of the device when the latter is within the secondary cooling chamber of the continuous casting machine; Fig.4 shows a semi-logarithmic diagram of a possible development of the surface temperature and internal temperature of the bloom as a function of time. The reference number 10-110 in the attached figures denotes generally a spray box device according to the invention. The spray box device 10-110 according to the invention is fitted to a continuous casting line 11 comprising in this case a tundish 12, a mould 13, a secondary cooling chamber 14, an extraction and straightening assembly 15 and a shearing assembly 17. The spray box device 10 according to the invention as applied to the line of Fig.1a has the purpose of cooling continuously a bloom 19 leaving the extraction and straightening assembly 15 so as to carry out a surface quenching of the outer layer of the bloom 19 in a controlled manner. The spray box device 10 according to the invention is fitted immediately downstream of the extraction and straightening assembly 15 and upstream of the shearing assembly 17. According to a variant a spray box device 110 is included which is fitted upstream of the extraction and straightening assembly 15 and within the secondary cooling chamber 14 of the continuous casting machine. This spray box device 110 can be fitted as an alternative to, or in combination with, the spray box device 10. According to the variant of Fig.1b, in which a short secondary cooling chamber 14a is included which does not reach the vicinity of the extraction and straightening assembly 15, the spray box device 10 is fitted upstream of the extraction and straightening assembly 15 but in a position outside the secondary cooling chamber 14a. The spray box device 10-110 according to the invention comprises a plurality of sprayer nozzles 18 arranged about the periphery of the bloom 19 and aimed at the bloom 19 being fed. In the case at least of the spray box device 10, the sprayer nozzles 18 are arranged within a containing box structure 37; this containing box structure 37 may be not included in the case of the spray box device 110 (Fig.3c) located within the secondary cooling chamber 14 of the continuous casting machine. The sprayer nozzles 18 may be arranged in a plurality of rows 20 positioned longitudinally so as to lap a segment of a determined length of the bloom 19. In this case the sprayer nozzles 18 are associated with a feeder manifold 21, which is connected to means 22 delivering water under pressure. The feeder manifold 21 includes advantageously means 23 to regulate the water pressure and means 24 to regulate the rate of flow of water so that these two parameters can be altered according to the type of material and the variations of the speed of the bloom 19 being fed with a view to ensuring constant cooling. According to a variant the pressure regulation means 23 and flow rate regulation means 24 are positioned in such a way that the sprayer nozzles 18 can be fed in a differentiated manner to suit requirements. According to another variant the sprayer nozzles 18 are divided into groups 118 in a transverse direction and/or lengthwise direction and/or according to the face of the bloom 19 which they are facing; these groups 118 of sprayer nozzles 18 are associated with their own feeder manifold 21 connected to the means 22 delivering water under pressure. This differentiation of pressure and/or rate of flow of the cooling liquid delivered by the sprayer nozzles 18-118 may be called for, for instance, where it is desired to produce a bloom 19 with characteristics which are not uniform. The speed of the bloom 19 is measured advantageously by speed measurement means 16 fitted to the continuous casting line 11. The pressure and rate of flow of the water fed to the sprayer nozzles 18 can be regulated also according to the properties of the steel, the speed of feed of the bloom 19 and the temperature of the bloom 19 so as to obtain at the desired temperature a precise cooled layer which is then tempered according to a determined curve. Moreover, the pressure and rate of flow of the water fed to the sprayer nozzles 18 can be regulated also according to the type of heating and rolling process which the bloom 19 is to undergo thereafter. The spray box device 10-110 according to the invention can be associated advantageously with a controlling, programming and governing unit 25 which governs the means 23-24 that regulate the pressure and rate of flow. According to a variant means 26 to measure the temperature of the bloom 19 are included and are fitted immediately upstream 26a and immediately downstream 26b respectively of the spray box device 10-110 according to the invention. The means 26 to measure the temperature of the bloom 19 are associated advantageously with the controlling, programming and governing unit 25 for automatic regulation of the spray box device 10-110 according to the invention. The controlling, programming and governing unit 25 comprises advantageously storage means 33 containing technological data relating to the various types of steels and the working parameters of the spray box device 10-110 according to the invention. The controlling, programming and governing unit 25 is associated advantageously with data input means 34 which comprise a keyboard for instance. The spray box device 10 includes, at its inlet and outlet, means 27 to deliver air under pressure so as to prevent the emerging, from the box structure 37, of steam generated by contact between the cooling water and the bloom 19. These means 27 to deliver air under pressure are arranged to create a wall of air directed substantially at a right angle to the bloom 19 being fed. This wall of air acts to close the spray box device 10 according to the invention and has the task of reducing the emerging, from the box structure 37, of steam released within the spray box device 10 according to the invention. The wall of air arranged at the outlet section of the spray box device 10 according to the invention has the further task of removing the water which tends to stay on the surface of the bloom 19 and which could lead to localised and uncontrolled undercooling of the surface of the bloom 19 with the resulting occurrence of a lack of homogeneity of the cooling. The means 27 to deliver compressed air are fed in this case by a pipe 28 associated with an aspiration means 29 and including at the end of the pipe 28 a filter means 30, which is advantageously of a replaceable type. In this case an aspiration hood 31 fitted above the spray box device 10 is included to aspirate and discharge steam leaving the spray box device 10. The aspiration hood 31 may be not included in cooperation with the spray box device 110 positioned inside the secondary cooling chamber 14. In this case the bloom 19 leaving the spray box device 10 positioned downstream of the extraction and straightening assembly 15 cooperates downstream with an insulated hood 32, which by accelerating the self-tempering of the bloom 19 assists the shearing operations carried out by the shearing assembly 17 positioned downstream. The insulated hood 32 may include its own heating means consisting, for instance, of burners, which are not shown here. According to a variant this insulated hood 32 extends beyond the shearing assembly 17. As an example, the semi-logarithmic diagram shown in Fig.4 represents the momentary development of the temperatures on the surface 36 and in the core 35 respectively of the bloom 19 subjected to the surface quenching treatment in the spray box device 10 according to the invention.

Claims: Method for the thermal surface treatment, in line, of blooms of fine-grain structural steels in a continuous casting machine associated to a heating furnace to heat hot-charge blooms, to obviate to the precipitation of compounds of aluminium, vanadium, niobium and the like and to eliminate or at least to greatly reduce the surface faults due to tension, wherein said continuous casting machine comprises a mould (13), a secondary cooling chamber (14), an extraction and straightening assembly (15) and a shearing assembly (17), wherein said method comprises a first cooling step of said blooms (19) within said secondary cooling chamber (14) and a second cooling step of said blooms (19) before said shearing assembly (17), said method being characterised by the step of performing a surface quenching of the outer layer of said blooms (19) by means of said second cooling step which is obtained with an intense and concentrated cooling of the surface of said blooms (19) to reduce the surface temperature of said blooms (19) between about 400°C and about 900°C after the natural tempering caused by the hot core of the blooms (19), said intense and concentrated cooling being realised by spraying, through a plurality of sprayer nozzles (18), a water-based cooling fluid under pressure against the surface of said blooms (19), wherein said intense and concentrated cooling is function of the dimensions of said blooms (19) and wherein said intense and concentrated cooling is applied immediately before or immediately after the step of extraction and straightening of said blooms (19) by said extraction and straightening assembly (15). '- Method as in claim 1, wherein the thickness of the outer layer of said blooms (19) affected by said intense and concentrated cooling is at least one centimetre. Method as in claim 1 or 2, wherein said blooms (19) are heated in said heating furnace with a speed of heating reaching up to 500°C per hour. Method as in any claim hereinbefore, wherein said intense and concentrated cooling is applied immediately upstream of said extraction and straightening assembly (15). Method as in any claim from 1 to 3, wherein said intense and concentrated cooling is applied within said secondary cooling chamber (14). Method as in any claim from 1 to 3, wherein said intense and concentrated cooling is applied immediately downstream of said extraction and straightening assembly (15). Method as in any claim hereinbefore, further comprising the step of lapping said blooms (19) with a current of air under pressure, before said cooling step. Method as in any claim hereinbefore, further comprising the step of lapping said blooms (19) with a current of air under pressure, after said cooling step. Method as in any claim hereinbefore, further comprising the step of passing said blooms (19) below an insulated hood (32), after said second cooling step. Method as in any claim hereinbefore, wherein at least said second cooling step is governed by a controlling, programming and governing unit (25), which is associated with at least means (16) measuring the speed of said blooms (19) and with means (26) measuring the temperature of said blooms (19) at the inlet (26a) and outlet (26b) of a spray box (10) wherein said intense and concentrated cooling is performed. Method as in claim 10, wherein said controlling, programming and governing unit (25) governs means (24) that regulate the rate of flow of said cooling fluid. Method as in claim 10 or 11, wherein said controlling, programming and governing unit (25) governs means (23) that regulate the pressure of said cooling fluid. Spray box device to carry out a thermal surface treatment, in line, of blooms of fine-grain structural steels to obviate to the precipitation of compounds of aluminium, vanadium, niobium and the like and to eliminate or at least to greatly reduce the surface faults due to tension, wherein said device is disposed in a continuous casting machine associated to a heating furnace to heat hot-charge blooms, wherein said continuous casting machine includes at least a mould (13), a secondary cooling chamber (14) for performing a first cooling of said blooms (19), an extraction and straightening assembly (15) and a shearing assembly (17), said device comprising a plurality of sprayer nozzles (18) for performing a second cooling of said bloom (19) before said shearing assembly (17), said sprayer nozzles (18) being fed by pump means (22) delivering a water-based cooling fluid under pressure and associated at least with means (23) that regulate said pressure, said device being characterised in that said plurality of sprayer nozzles (18) is disposed around the periphery of the blooms (19) and facing the surfaces thereof, in that said sprayer nozzles (18) are able to perform an intense and concentrated cooling of the surface of said bloom (19) for performing a surface quenching of the outer layer of said blooms (19) by reducing the surface temperature of said blooms (19) between about 400°C and about 900°C after the natural tempering caused by the hot core of the blooms (19), wherein said intense and concentrated cooling is function of the dimensions of said blooms (19), and wherein said device is disposed immediately before or immediately after said extraction and straightening assembly (15). Spray box device as in claim 13, wherein said pump means (22) are associated with means (24) that regulate the rate of flow of said cooling fluid. Spray box device as in claim 13, further comprising an inlet (26a) and an outlet (26b), temperature measuring means (26) disposed at said inlet (26a) and said outlet (26b) for measuring the temperature of said blooms (19), and a controlling, programming and governing unit (25) associated with said temperature measuring means (26) and with means (16) that measure the speed of said blooms (19). Spray box device as in claim 15, wherein said controlling, programming and governing unit (25) comprises storage means (33) wherein are stored the information on the properties of the various types of steels of said blooms (19) and the working parameters such as the thickness of said blooms (19) to be treated and the temperatures to which said blooms (19) have to be brought and thereafter in tempering for the purpose of keeping constant the thickness of the cooled layer and the cooling and tempering temperature along the whole blooms (19). Spray box device as in claim 15 or 16, wherein said controlling, programming and governing unit (25) is associated with data input means (34). Spray box device as in claim 13, wherein means (27) for delivering compressed air are provided at the inlet and/or at the outlet thereof. Spray box device as in claim 13, wherein an insulated hood (32) is positioned immediately downstream of the outlet thereof. Spray box device as in claim 13, further comprising an aspiration hood (31). Spray box device as in claim 13, wherein it is disposed immediately downstream of said extraction and straightening assembly (15) and upstream of said shearing assembly (17). Spray box device as in claim 13, wherein it is disposed immediately upstream of said extraction and straightening assembly (15). Spray box device as in claim 13, wherein it is disposed within said secondary cooling chamber (14).

Inventors: BOMBARDELLI BRUNO, TERCELLI CRISTIANO, BOMBARDELLI, BRUNO, TERCELLI, CRISTIANO
Applicants: DANIELI OFF MECC, DANIELI & C. OFFICINE MECCANICHE S.P.A.
Application year: 1994
Main classification: B22D11
IPC codes: B22D11